The onset of a cooler climate 4 million years ago — rather than tectonics — helped carve deep canyons in the Andes, geologists report in the Aug. 16 Science.

The researchers examined one of the many 1.5- to 2.5-kilometer-deep canyons that cut into the northeastern edge of the Andean Plateau in southern Peru. By dating when rocks at various depths within the chasm were exposed to the surface, the team determined that rapid erosion created the canyon.

Geologists usually tie canyon formation to uplift triggered by tectonic activity in Earth’s crust or movement in the mantle. Once the surface rises, erosion usually starts right away, says coauthor Richard Lease, now at the U.S. Geological Survey in Anchorage, Alaska. But in this case, the region was in a tectonic lull when erosion began, Lease and colleague Todd Ehlers of Germany’s University of Tübingen say.

Instead, they suggest, climate change activated canyon formation. Roughly 4 million years ago, cooling in the Pacific Ocean initiated periodic La Niña events that intensified storms in the region. Cooling in the North Atlantic would also have caused more rainfall, and thus more erosion.

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